# How to make a motor capable of resisting physical movement?

Is it possible to make a motor capable of resisting physical movement?

For example, at the gym you control the resistance to movement by putting more load, more weight. Is there a way to control that resistance electronically? Maybe by using a motor controller adjusting the PWM signal?

• Typically you build a servo control system which acts (with limited power) to try to prevent movement. – Chris Stratton Apr 29 at 10:39
• You could, but its a lot more complicated and expensive than the non-motor methods that already exist, such as those used in rowing machines or exercise bikes: water, magnetic, fan, etc. I would also be paranoid about malfunction since the same system can rip your arms off. A lot of safety issues. – DKNguyen Apr 29 at 14:23

You could use a generator as a variable load. Use a PWM signal to short circuit the output of the generator. Shorter duty cycle is less load, longer duty cycle is more load.

A generator is just a motor whose shaft is turned by an external force.

• even with a Motor Controller connected is this possible? am i going to short circuit the output of the motor controller then? – Julius Noel Banayo Apr 30 at 2:21

Somethink like that. If you wan't a constant torque controller then you should measure the motor current and adjust PWM duty ratio so that the motor current is equal or less the current setpoint (torque setpoint).

simulate this circuit – Schematic created using CircuitLab

The simplest method, which you will find in a number of stair-stepper, elliptical and stationary bicycles at the gym is to use an automotive alternator. These have the advantage of a wound field, so it's possible to vary the field current, which will be much lower than the output current, by either changing the voltage applied, or use of PWM from a fixed supply, and dissipating the generated output in a load resistor. This also has the advantage of being able to generate the supply for the control electronics and display.

With a permanent magnet machine, you would have to control the generated current, with what would effectively be a controllable constant current load.

This works for rotary motion, and for reciprocating or linear motion you can arrange some kind of linkage to translate that to rotary - the stair steppers use a chain drive. Alternatively, you could use a eddy current or hysteresis brake, where the moving element (aluminum or copper for eddy current, steel for hysteresis) passes through the magnetic field generated by an electromagnet. Transformer E laminations work well for making these.